Mixing processes are widely employed in unit operations intended to make heterogeneous physical systems more homogenous. A typical approach is the use of axial or radial impellors in agitated tanks for the mixing of fluids or slurries. These tanks utilize shear imparted to the fluid or slurry by the impellor to generate circulating flows within the tank. Typically the fluids or slurries mixed are non-hazardous materials, and draining and opening the tank to correct various issues arising with any moving parts inside the vessel is an acceptable method of repair.
When the mixing tanks are intended for the storage of relatively hazardous materials, such as radioactive waste awaiting remediation, draining and opening the tank becomes a significantly more complicated undertaking. However, mixing of the waste during storage remains a significant operational requirement, because typically the radioactive waste is made up of solid particles and liquids, and particles with high concentrations of fissile materials such as uranium or plutonium are dense, rapidly settling particles. In the absence of adequate mixing during storage, the rapidly settling particles can settle preferentially and accumulate, creating a potential for inadvertent criticality in the sediment. Additionally, the sediment layers can retain significant quantities of flammable gas generated from radiolysis, which, without adequate mixing, may be released suddenly through a spontaneous buoyant displacement gas release event and potentially exceed the lower flammability limit in the mixing vessel headspace.
Due to the requirement for periodic mixing, combined with the logistical difficulties of opening a radioactive waste tank for repair, it is generally desired that radioactive storage tanks provide a mixing capability with an absence of any moving parts within the tank itself. To meet these requirements, fluidic pulse jet mixers (PJM) are commonly employed. PJM's employ pulse jets formed by alternating pressure and suction on fluid in pulse tubes coupled to jet nozzles, creating a pulsating flow. The nozzle end of the tube is immersed in the tank, while periodic pressure, vacuum, and venting are supplied to the opposite end. A suction phase draws process liquid into the PJM from the vessel, and a drive phase subsequently pressurizes the PJM with compressed air. This pressurization discharges the PJM liquid at high velocity into the vessel, causing mixing to occur. The drive phase is followed by a vent phase, which allows for depressurization of the PJM. These three phases (suction, drive, and vent) make up the mixing cycle. Such a system is intended to provide a motive mixing force within the tank without reliance on moving parts within the tank environment.
Pulse jet mixers as described are commonly used, however certain undesired characteristics remain. Generally speaking, during the drive phase, flow from the jet moves radially away from the jet into the waste sediment, the flow velocity decreases, and the drag force per particle decreases with the increasing radius. This entrains new particles, however the slowing particles subject to the decreasing flow velocity are not removed from the flow, and the particles bunch together, so that multiple particles form large masses of particles. These large masses effectively act as a very large particle. The time constant for the large masses is long, and clumping and cratering within the waste sediment results. In the limiting case, acceleration of mass goes to zero and PJM energy is totally dissipated by the formation of stable crater walls. These dead zones are an undesired situation with regard to radioactive wastes for the reasons discussed above. PJMs typically expend significant energy ensuring that limiting cases are avoided.
Particle clumping is a natural consequence of decelerating multiphase flows, where fluid motion is relatively fast, particles are pushed, and the resulting particle motion is relatively slow. It would be advantageous to provide a pulse jet mixing vessel where mixing of solids could occur within the vessel without attendant moving parts, and where the mixing could occur through an accelerating, radial inward flow over a bed of particles. Such a flow would tend to pull particles into a central upwash jet and into the body of the fluid, greatly mitigating or eliminating the formation of sediment craters and any associated dead zones. It would be particularly advantageous if the pulse jet mixing vessel produced the inward radial flow in a manner which eliminates dissipative and unnecessary secondary flows, so that acceleration of inward radial flow over the leading edge and the bulk of the sediment would reduce fluid/solid shear transport mechanisms, greatly reduce the production of turbulent dissipation, enhance wake formation at the trailing edge, and enhance the transport mechanism of the central upwash fountain.
These and other objects, aspects, and advantages of the present disclosure will become better understood with reference to the accompanying description and claims.